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#1
09-09-2007, 10:12 AM
 flex727 Guest Join Date: Mar 2002 Location: Allen, TX Posts: 1,253
Non-overlapping WiFi channels?

According to this article, the only non-overlapping WiFi channels are 1, 6, & 11. But if you look at the accompanying diagram, it appears that all the channels overlap some other channels. Can anyone explain what is meant by the statement that channels 1, 6, & 11 are non-overlapping?
#2
09-09-2007, 11:14 AM
 jmwatts Guest Join Date: Jul 2002 Location: Austin, TX Posts: 75
It basically means that channels 1, 6, and 11 do not interfere with each other. (They actually do a tiny bit, but it's small enough that it can be ignored).
Each channel has 20MHz of bandwidth. If you look at the upper frequency of Ch 1 (2.423) you find that it is lower than the lowest frequency of channel 6 (2.426), hence they do not overlap. By the same token, channels 6 and 11 do not overlap.
If you look at channels 1 and 2, you see that they "share" quite a bit of the same frequency band. This can cause interference.
#3
09-09-2007, 11:22 AM
 Musicat Charter Member Join Date: Oct 1999 Location: Sturgeon Bay, WI USA Posts: 19,977
BTW, just a word to the wise...if you have interference with a 2.4Ghz cordless telephone, try changing the channel on your WiFi network(s). I had a client whose access to the Internet ceased whenever his phone was in use. I changed the WiFi channel from 6 to 1 and the problem went away.
#4
09-09-2007, 11:47 AM
 Fuzzy Dunlop Guest Join Date: Apr 2007 Posts: 3,457
Quote:
 Originally Posted by flex727 According to this article, the only non-overlapping WiFi channels are 1, 6, & 11. But if you look at the accompanying diagram, it appears that all the channels overlap some other channels. Can anyone explain what is meant by the statement that channels 1, 6, & 11 are non-overlapping?
What jmwatts said is completely correct. But if I understand you, it sounds like you`re confused by the fact that your chart clearly shows four trios of channels that don't overlap each other, but do overlap some channel. I think the important thing to take away is that the WiFi spectrum is wide enough for 3 non-overlapping 22 mhz channels. Since the frequency range starts at 2400 and channel 1, naturally, starts at 2400, it only makes sense to say that 1, 6, and 11 are the three non-overlapping channels.
#5
09-10-2007, 12:45 AM
 Musicat Charter Member Join Date: Oct 1999 Location: Sturgeon Bay, WI USA Posts: 19,977
Quote:
 Originally Posted by Chareth Cutestory ...your chart clearly shows four trios of channels that don't overlap each other, but do overlap some channel...
From the same link, "In the United States and Canada there are 11 channels available for use." Channels 12..14 are not available even tho they are shown on the chart.
#6
09-10-2007, 10:44 AM
 flex727 Guest Join Date: Mar 2002 Location: Allen, TX Posts: 1,253
Quote:
 Originally Posted by Chareth Cutestory What jmwatts said is completely correct. But if I understand you, it sounds like you`re confused by the fact that your chart clearly shows four trios of channels that don't overlap each other, but do overlap some channel. I think the important thing to take away is that the WiFi spectrum is wide enough for 3 non-overlapping 22 mhz channels. Since the frequency range starts at 2400 and channel 1, naturally, starts at 2400, it only makes sense to say that 1, 6, and 11 are the three non-overlapping channels.
I guess they're saying that if you need THREE non-overlapping channels, those would be the only ones, but are are 14 unique sets of 2 non-overlapping channels. Because you could have interference with another device, such as 2.4GHz cordless phones, or a neighbor's wireless network, 2 non-overlapping channels would be advantageous - what's the big deal about 3?
#7
09-10-2007, 10:48 AM
 citybadger Guest Join Date: Nov 2002 Location: NYC Posts: 181
Related Question:

If I have two (or more) wireless access points on the same network, just spread out to provide better coverage around the building, should I set them to different channels so they don't interfere with one another, or doesn't it matter since they are on the same network?
#8
09-10-2007, 12:28 PM
 Musicat Charter Member Join Date: Oct 1999 Location: Sturgeon Bay, WI USA Posts: 19,977
Quote:
 Originally Posted by citybadger Related Question: If I have two (or more) wireless access points on the same network, just spread out to provide better coverage around the building, should I set them to different channels so they don't interfere with one another, or doesn't it matter since they are on the same network?
Signals on the same (or overlapping) channels will interfere with each other if they are in range, so it's best to use different channels.

How much interference? Maybe not that much. I happen to work in an office with 4 separate signals that originate only a few feet from each other; all are on channel 6, and each is a different network. (I advised them to use different channels, but they didn't take my advice.) It seems to work. I suspect someone with an analyzer could detect collisions and retransmissions, but in practice, it isn't noticeable. Of course, none of the networks is gaming or streaming video, either, just low-intensity office stuff.

OTOH, in the example I gave earlier, a 2.4Ghz cordless phone stopped network communication on channel 6 cold while it was off-hook.

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